Traditional approaches for producing lactams, used in the production of nylon, include an oxime undergoing a Beckmann rearrangement in the presence of an acid catalyst, such as fuming sulfuric acid.
Oximes are compounds having the general formula:
wherein R1 is an organic group and R2 is hydrogen or an organic group. When R2 is hydrogen, the oxime is an oxime derived from an aldehyde, referred to as aldoximes. When R2 is an organic group, the oxime is an oxime derived from a ketone, referred to as ketoximes.
Cyclic oximes are a sub-group of ketoximes having the general formula:
wherein the R1 and R2 groups form a ring.
Lactams, or cyclic amides, are compounds having the general formula:
wherein R1 and R2 form a ring.
Exemplary oximes include, but are not limited, to cyclohexanone oxime, cyclododecanone oxime, 4-hydroxy acetophenone oxime and oximes formed from acetophenone, butryaldehyde, cyclopentanone, cycloheptanone, cyclooctanone, and benzaldehyde. Exemplary lactams include those made from cyclic oximes, including those listed above. Lactams are well known in the art as being useful in the production of polyamides, such as nylon. ε-caprolactam can be polymerized to form Nylon-6. ω-laurolactam can be polymerized to form Nylon-12. Additional examples of useful lactams include 11 undecanelactam, a precursor of Nylon-11, 2-Pyrrolidone a precursor of Nylon-4, 2-Piperidone a precursor of Nylon-5.
Exemplary reactions are shown in FIG. 1. As illustrated in FIG. 1A, cyclohexanone oxime is reacted to form ε-caprolactam. ε-caprolactam in turn is polymerized to form nylon-6. As illustrated in FIG. 1B, cyclododecanone oxime is reacted to form ω-laurolactam. ω-laurolactam in turn is polymerized to form nylon-12. As illustrated in FIG. 1C, cyclooctanone oxime is reacted to form the corresponding lactam (caprylolactam), which in turn can be polymerized to form nylon-8. Nylon-6, nylon-8, and nylon-12 are extensively used in industry and manufacturing.
One potential reaction mechanism for the reaction of FIG. 1A is illustrated in FIG. 1D. The mechanism generally consists of protonating the hydroxyl group, performing an alkyl migration while expelling the hydroxyl to form a nitrilium ion, followed by hydrolysis, tautomerization, and deprotonation to form the lactam.
Typically, Beckmann rearrangement reactions of oximes to form lactams are performed using acids such as fuming sulfuric acid. These reactions are characterized by complete or nearly complete conversion of the oxime and very high selectivity for the desired lactams. However, these reactions also produce byproducts including ammonium sulfate. Although ammonium sulfate is a useful product in itself, minimizing its production may be desirable.
Different catalysts, such as zeolites have been proposed for use in optimizing the Beckmann rearrangement. It is widely regarded that weak Brønsted sites are required and as such a range of different microporous catalysts, including zeolites, aluminophosphates (AlPO), metal substituted aluminophosphates (MeAlPO), and mesoporous catalysts, including MCM-41 and SBA-15 have been proposed. Zeolites, such as the highly siliceous MFI zeolite catalyst, ZSM-5, have been used in the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam.
However, typical microporous structures may include one or more disadvantages, including a drop in activity over time due to the formation of carbon deposits on the active sites that act as a poison, reduced mass transfer, diffusion limitations, reduced substrate versatility, and limitations on pore size. Zeotypes having large pores, such as AlPO-8 (AET), VPI-5 (VFI), and cloverite (CLO) may include terminal hydroxyl groups, reducing the stability of the structure. Moreover, these larger pored zeotypes may include strong acid sites, which are less favorable for certain types of reactions, and may not result in increased versatility, longevity, and activity. Mesoporous silicas and isomorphously substituted metals in mesoporous systems, such as Mg-MCM41, Al-MCM41, and MgAl-MCM41, may be less stable, less selective, and less active than microporous catalysts, and their amorphous framework may result in reduced stability.
Improvements in the foregoing processes are desired.